Add a non-interactive mode to tinc commands

Some tinc commands, such as `tinc generate-keys`, use the terminal to ask the user for information. This can be bypassed by making sure there is no terminal, which is trivial on *nix but might require jumping through some hoops on Windows depending on how the command is invoked.

This commit adds a `--batch` option that ensures tinc will never ask the user for input, even if it is attached to a terminal.